The present invention relates to an optical element having concave and convex portions as parallel grooves on the surface of a substrate and, particularly, to a diffraction grating and an optical waveguide element.
Optical parts such as a diffraction grating and an optical waveguide are combined together to serve as an integrated optical element for transmitting light by convergence or diffusion.
As for the diffraction grating, the following diffraction gratings (1) to (6) are known.
(1) Diffraction gratings which are a laminate consisting of glass sheets with a thickness difference of xe2x80x9cdxe2x80x9d arranged in tiers (refer to A. A. Michelson: Astrophys. J. no. 8, pp. 36, 1893).
(2) Diffraction gratings produced by precision machining, silicon photolithography and selective etching (Donald H. McMaon, Applied Optics, vol. 26, no. 11, pp. 2188 (1987), JP-A 63-33714, U.S. Pat. No. 4,736,360) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
(3) Diffraction gratings produced by a method in which a hydrolysis solution prepared from a metal alcoholate is pressed against a transfer mold and cured by heat and light (JP-A 62-102445, JP-A 62-225273 and JP-A 10-142410). Out of these, JP-A 62-102445 discloses a process for producing a diffraction grating by a so-called sol-gel method in which a solution containing silicon alkoxide is applied to the surface of a glass substrate and heated while a mold having concave and convex portions is pressed against the solution to form concave and convex portions.
(4) There is known a method (refer to JP-A 63-49702) in which an ultraviolet curable resin is uniformly spread over a substrate and irradiated with ultraviolet radiation while a mold having concave and convex portions is pressed against the resin.
(5) JP-A 6-242303 discloses a method for forming a plurality of layers on a substrate when a film as thick as more than several micrometers is to be formed by a sol-gel method. In this case, constituents for each layer are prepared in the form of a solution or sol, pressurized and heated while a mold is pressed against the solution or sol, completely solidified as a layer, and then a solution or sol is further poured over the solidified layer to form an upper layer.
(6) J. Am. Ceram. Soc., vol. 81, no. 11, pp. 2849 to 2852 (1998) discloses a method for producing an optical disk having a fine groove structure by applying a solution containing methyl triethoxysilane and tetraethoxysilane to the surface of a substrate.
As for the optical waveguide element, besides those produced from inorganic materials, there are known optical waveguide elements produced from organic materials such as
(7) PMMA (Ryoko Yoshimura. J. of Lightwave Technology, vol. 16, no. 6 (1998)), (8) a polyimide and (9) a silicon-based polymer (Mitsuo Usui, J. of Lightwave Technology, vol. 14, no. 10 (1996)) by photolithography and etching.
However, the above prior arts have the following problems. First, it cannot be said that the above method (1) has excellent productivity because a technology for laminating thin glass sheets precisely is extremely difficult. Further, it is impossible to apply the method to a diffraction grating having a lens function like a concave grating.
The method (2) makes use of photolithography and needs a complicated step. The surface of the obtained diffraction grating has a shape depending on the surface of a crystal grating forming a diffraction grating and the height of each step can be set only to a limited range in fact.
The method (3) has such a problem that the thickness of a film forming a diffraction grating is several micrometers or less and diffraction efficiency greatly depends on the polarization of light as the cycle and height of concave and convex portions forming the diffraction grating are close to the wavelength of light used for optical communication, for example, 1.3 xcexcm or 1.55 xcexcm. When the thickness of the film is made large enough to form a diffraction grating having a larger step height than the wavelength of light in order to reduce dependence on polarization, the film easily cracks and is inferior in heat resistance.
As for the method (4), the ultraviolet curable resin decomposes or yellows at a temperature higher than 250xc2x0 C. due to its low heat resistance. Therefore, the substrate having concave and convex portions of the ultraviolet curable resin cannot be soldered and difficult to be attached to a device.
The method (5) is a method for forming a laminate by molding organopolysiloxane layers one after another, which makes it possible to form an organopolysiloxane layer having concave and convex surface and a thickness of several tens of micrometers. However, the production process is long, thereby boosting costs. Further, since the next layer is formed after an underlying layer is completely cured, undesired air is contained between a mold and a solution or sol with the result of low dimensional accuracy of concave and convex portions.
Further, the method (6) is capable of producing an optical disk having a sol-gel film with a maximum film thickness of less than 300 nm. However, when the formed film is heated at a temperature required for soldering, for example, 350xc2x0 C. and then cooled to form a diffraction grating having a film thickness of 500 nm to several micrometers, the film cracks.
Meanwhile, as for the optical waveguide, optical waveguide elements made from inorganic materials have high reliability but cannot be mass-produced at a low cost. As for optical waveguide elements made from the above organic materials (7) to (9), there are few materials which are satisfactory in terms of heat resistance and a complicated step such as photolithography or etching is needed to form a core portion for transmitting light.
The present invention has been made in view of the above problems of the prior art.
It is therefore an object of the present invention to provide an echelon diffraction grating which can be produced at a low cost and has excellent heat resistance.
It is another object of the present invention to provide an optical waveguide which has high heat resistance and small absorption at a communication wavelength of a near infrared range, and satisfies reliability and a low loss at an optical communication band by a simple production process.
Other objects and advantages of the present invention will become apparent from the following description.
Firstly, according to the present invention, the above objects and advantages of the present invention are attained by an echelon diffraction grating which comprises a substrate and an organopolysiloxane film having a maximum thickness of 1 xcexcm to 1 mm formed on the surface of the substrate, and has a plurality of steps having a predetermined height of 5 to 500 xcexcm and a predetermined width of 1 to 500 xcexcm formed on the organopolysiloxane film, wherein
the organopolysiloxane film contains dimethylsiloxane represented by the following chemical formula (1) and a phenylsiloxane represented by the following chemical formula (2) as essential ingredients:
(CH3)2SiO2/2xe2x80x83xe2x80x83(1)
PhSiO3/2xe2x80x83xe2x80x83(2)
wherein Ph is a phenyl group or substituted phenyl group.
The predetermined height and the predetermined width correspond to xe2x80x9cdxe2x80x9d and xe2x80x9cwxe2x80x9d in the attached FIG. 2, respectively.
Secondly, according to the present invention, the above objects and advantages of the present invention are attained by an optical waveguide element which comprises a core portion for transmitting light, formed on the surface of a substrate and having a height of 1 to 30 xcexcm and a width of 1 to 30 xcexcm and length extending along the surface of the substrate, and a clad portion surrounding the core portion or interposed between the core portion and the substrate, wherein
at least one of the core portion and the clad portion contains dimethylsiloxane represented by the following chemical formula (1) and a phenylsiloxane represented by the following chemical formula (2) as essential ingredients:
(CH3)2SiO2/2xe2x80x83xe2x80x83(1)
PhSiO3/2xe2x80x83xe2x80x83(2)
wherein Ph is a phenyl group or substituted phenyl group.